Superconductivity and structure of a few-unit-cells-thick Bi-Sr-Ca-Cu-O ultrathin films

Bi-Sr-Ca-Cu-O ultrathin films with thicknesses below 100 Å down to 10 Å have been deposited just on the (001) MgO substrate by single target rf magnetron sputtering. By elaborately controlled multistep annealing for extremely short times, remarkably high values of the zero-resistance transition temperature, $T_{c,0\mathrm{}},$ 106, 88, and 84 K were obtained for 70-, 40-, and 20-Å-thick ultrathin films, respectively. A 10-Å-thick ultrathin film was almost insulating. Cross-sectional high-resolution transmission electron microscopy revealed that a 40-Å-thick ultrathin film with a $T_{c,0\mathrm{}}$ of 88 K consisted of a set of half-unit-cell layers of the (2212) and (2223) phases. A 20-Å-thick ultrathin film consisted of a half-unit-cell layer of the (2212) phase. Thus, it was confirmed that in the Bi system the minimum unit for the occurrence of superconductivity is a half-unit-cell layer of the superconducting oxide crystal. Instead of the repeated use of thermal treatment at high temperatures, ion irradiation techniques combined with annealing at relatively low temperatures was useful to modify the crystal quality and related transport properties of our ultrathin films.